This invention relates in general to superconducting electrical conductors and, more specifically, to a tube having a high temperature superconductor formed along a helical internal path.
Materials which have superconducting properties; that is, have essentially no resistance at very low temperatures, have been known for some time. Superconductors have been used in a number of devices that take advantage of two unique properties of superconductivity, low (ideally zero) power consumption and the compactness possible with superconductors because of the absence of power dissipation losses and their high current density.
Ductile alloys of niobium and tin, titanium, tantalum, zirconium and other metals have been used in electromagnets in medical apparatus, experimental devices in physics, for energy generation in magnetohydrodynamic generators and are proposed for use in levitating trains and many other purposes. Unfortunately, these materials are superconducting only at temperatures below about 20K. and must be constantly cooled by liquid helium during use. Liquid helium is expensive and difficult to handle.
A number of "high temperature" superconducting ceramic materials were discovered in the late 1980's. "High temperature" is a relative term and now is generally taken to mean any temperature above the boiling temperature of liquid nitrogen, 78.degree. K. The best known of these new superconductors contain one atom of a rare earth metal such as yttrium, two barium atoms, three copper atoms and about seven oxygen atoms and is referred to as a "1-2-3" superconductor. Other high temperature superconductor systems include bismuth-strontium-calcium-copper-oxide and thallium-barium-calcium-copper-oxide materials. It is expected that additional ceramic superconducting materials will be discovered.
While these high temperature superconductors have varying desirable characteristics, primary of which is the high superconducting transition temperature, they also have a number of drawbacks limiting their rapid introduction into commercial products. A major problem is material brittleness and lack of ductility, which prevents the convenient production of wires, filaments and the like. Also, these materials tend to have low critical current density, often less than 1000 A/sq. cm. These problems have prevented the use of these materials in such applications as large superconducting magnet coils and power transmission lines.
Thus, there is a continuing need for improved methods and apparatus which permit the use of the high temperature superconductors in such devices.